Automatic gas control system for a gas discharge laser

ABSTRACT

An automatic F 2  laser gas control, for a modular high repetition rate ultraviolet gas discharge laser. The laser gas control includes techniques, monitors, and processor for monitoring the F 2  consumption rates through the operating life of the laser system. These consumption rates are used by a processor programmed with an algorithm to determine F 2  injections needed to maintain laser beam quality within a delivery range. Preferred embodiments include F 2  controls for a two-chamber MOPA laser system.

[0001] This application claims the benefit of U.S. provisionalapplication Serial No. 60/429,493 filed on Nov. 27, 2002 and the presentinvention is a continuation-in-part of Ser. No. 10/141,216 filed May 7,2002, of Ser. No. 10/036,676, filed Dec. 21, 2001, Ser. No. 10/036,727filed Dec. 21, 2001, Ser. No. 10/006,913 filed Nov. 29, 2001, and Ser.No. 09/943,343, filed Aug. 29, 2001, all of which are incorporatedherein by reference. This invention relates to lithography light sourcesfor integrate circuit manufacture and especially to gas discharge laserlithography light sources for integrated circuit manufacture.

PRIOR ART Gas Control Techniques

[0002] The prior art includes several techniques for automatic controlof the laser gas for discharge lasers such as excimer lasers. Forexample, U.S. Pat. No. 5,440,578 describes a gas control technique formaintaining Kr in a KrF laser. U.S. Pat. No. 5,978,406 describestechniques for monitoring the F₂ level and controlling the level with afeedback technique. U.S. Pat. No. 6,028,880 describes a technique usinga manifold to permit small precise injections. U.S. Pat. No. 6,151,349describes an F₂ injection technique based on measured values of dE/dV.U.S. Pat. No. 6,240,117 describes another F₂ injection techniqueutilizing an F₂ monitor.

Uses

[0003] An important use of excimer lasers is to provide the light sourcefor integrated circuit lithography. The type of excimer laser currentlybeing used in substantial numbers for integrated circuit lithography isthe KrF laser, which produces ultraviolet light at a wavelength of 248nm. A similar excimer laser, the ArF laser, provides ultraviolet lightat 193 nm, and an F₂ laser operates at 157 nm. These lasers typicallyoperate in a pulse mode at pulse rates such as 1,000 Hz to 4000 Hz. Thelaser beam is produced in a laser chamber containing a gain mediumcreated by an electric discharge through a laser gas between twoelongated electrodes of about 28 inches in length and separated by about{fraction (5/8)} inch. The discharge is produced by imposing a highvoltage such as about 20,000 volts across the electrodes. For the KrFlaser, the laser gas is typically about 1% krypton, 0.1% fluorine andabout 99% neon. For the ArF laser the gas is typically about 3 to 4%argon, 0.1% fluorine and 96 to 97% neon. The F₂ laser is about 0.15% F₂and the rest He. The laser gas must be circulated between the electrodesat speeds high enough to clear the discharge region between discharges.

Fluorine Depletion

[0004] Fluorine is the most reactive element known and it becomes evenmore reactive when ionized during the electric discharge. Special caremust be exercised to utilize in these laser chambers materials such asnickel coated aluminum which are reasonably compatible with fluorine.Further, laser chambers are pretreated with fluorine to createpassification layers on the inside of the laser chamber walls. However,even with this special care, fluorine will react with the walls andother laser components producing metal fluoride contaminants andresulting in a relatively regular depletion of the fluorine gas. Therates of depletion are dependent on many factors, but for a given laserat a particular time in its useful life, the rates of depletion dependprimarily on the pulse rate and load factor if the laser is operating.If the laser is not operating, the depletion rate is substantiallyreduced. The rate of depletion is further reduced if the gas is notbeing circulated. To make up for this depletion, new fluorine istypically injected at intervals of about 1 to 3 hours. Rather thaninject pure fluorine it is a typical practice to inject into KrF lasersa mixture of 1% fluorine, 1% krypton and 98% neon. For example, in aspecific high quality 1000 Hz KrF excimer laser used for lithography,the quantity of its fluorine, krypton, neon mixture injected tocompensate for the fluorine depletion varies from about 5 scc per hourwhen the laser is not operating and the laser gas is not beingcirculated to about 180 scc per hour when the laser is runningcontinuously at 1000 Hz. The typical injection rate is about 10 scc perhour when the chamber fan is circulating the laser gas, but the laser isnot firing.

[0005] The unit “sce” refers to “standard cubic centimeters”. Othercommonly used units for describing quantities of fluorine in aparticular volume are percent (%) fluorine, parts per million and kiloPascals; the latter term sometimes refers to the partial pressure of thefluorine gas mixture. (This is because the amount of fluorine injectedinto a laser chamber is typically determined (directly or indirectly) bythe measured chamber pressure increase while the 1% fluorine gas mixtureis being injected.) A 195 scc per hour injection rate of the 1% fluorinemixture would correspond to a depletion in the fluorine concentrationover 2 hours from about 0.10 percent to about 0.087 percent. The actualquantity of fluorine depleted in two hours as measured in grams of purefluorine would be about 17 milligrams during the two hour periodcorresponding to the above 320 scc/hour injection rate (i.e., 390 sec ofthe 1% fluorine mixture injected at two-hour intervals) of the fluorinegas mixture.

Modes of Operation

[0006] For integrated circuit lithography a typical mode of operationrequires laser pulses of constant pulse energy such as 10 mJ/pulse atabout 1000 Hz which are applied to wafers in bursts such as about 300pulses (with a duration of about 300 milliseconds) with a dead time of afraction of a second to a few seconds between bursts. Modes of operationmay be continuous 24 hours per day, seven days per week for severalmonths, with scheduled down time for maintenance and repairs of, forexample, 8 hours once per week or once every two weeks. Therefore, theselasers must be very reliable and substantially trouble-free.

[0007] In typical KrF and ArF excimer lasers used for lithography, highquality reproducible pulses with desired pulse energies of about 10mJ/pulse for KrF and 5 mJ/pulse for ArF may be obtained over asubstantial range of fluorine concentration (for example, from about0.08 percent to about 0.12 percent for KrF). Over the normal laseroperating range the discharge voltage required to produce the desiredpulse energy increases as the fluorine concentration decreases (assumingother laser parameters remain approximately constant). FIG. 1 shows thetypical relationship between discharge voltage and fluorineconcentration for constant pulse energy of 10 mJ and 14 mJ. Thedischarge voltage in the range of 15 kv to 20 kv is typically controlledby a feedback system which calculates a charging voltage (in the rangeof about 550 volts to 800 volts) needed to produce (in a pulsecompression-amplification circuit) the needed discharge voltage which isneeded to produce the desired laser pulse energy. This feedback circuittherefore sends a “command voltage” signal a power supply to providecharging voltage pulses.

PRIOR ART F₂ Control Techniques

[0008] Prior art techniques typically utilize the relationship betweendischarge voltage and fluorine concentration to maintain constant pulseenergy despite the continuous depletion of fluorine. The dischargevoltage of prior art excimer lasers can be changed very quickly andaccurately and can be controlled with electronic feedback to maintainconstant pulse energy. Accurate and precise control of fluorineconcentration in the past has proven difficult. Therefore, in typicalprior art KrF and ArF laser systems, the fluorine concentration isallowed to decrease for periods of about 1 to 4 or 5 hours while thedischarge voltage is adjusted by a feedback control system to maintainconstant pulse energy output. Periodically at intervals of about onehour to a few hours, fluorine is injected during short injection periodsof a few seconds. Thus, in normal operations fluorine concentrationgradually decreases from (for example) a ArF laser, about 0.10 percentto about 0.09 percent over a period of about 1 to a few hours while thecharging voltage is increased over the same period from, for example,about 600 volts to about 640 volts. The injection of fluorine at the endof the 1 to a few hour period (when the voltage has drifted up to about640 volts) brings the fluorine concentration back to about 0.10 percentand the feedback control (maintaining constant pulse energy)automatically reduces the voltage back to 600 volts. This basic processis typically repeated for several days. Injections are typicallyperformed automatically as controlled by a controller based on speciallycrafted control algorithms. As shown in FIG. 2, prior art excimer laserstypically divert a portion of the chamber gas flow through a metalfluoride trap to remove contamination. Laser beam 2 is produced in again medium between electrodes 4 (only the top electrode is shown inFIG. 2) in chamber 6 in a resonance cavity defined by line narrowingmodule 8 and output coupler 10. Laser gas is circulated between theelectrodes 4 by tangential blower 12. A small portion of the circulatingflow is extracted at port 14 downstream of blower 12 and directedthrough metal fluoride trap 16 and clean gas is circulated back into thechamber through window housings 18 and 20 to keep the windows free oflaser debris. A very small portion of each laser pulse is sampled bybeam splitter 22 and pulse energy monitor 24, and in an extremely fastfeedback control loop, controller 26 controls the electrode dischargevoltage to maintain pulse energy within a desired range by regulating ahigh voltage charging circuit 28 which provides charging current tovoltage compression and amplification circuit 30 which in turn providesvery high voltage electrical pulses across electrodes 4. Over a longerterm controller 26 through gas controller 27 also controls the fluorineconcentration in the chamber 6 by regulating fluorine injections atcontrol valve 32. Special control algorithms will periodically injectpredetermined quantities of fluorine. These injections may be called forwhen the high voltage has increased to a predetermined limit orinjection may be made after a predetermined number of pulses such as 3million pulses or after the passage of a predetermined period of time(such as six hours with no lasing) or a combination of pulses, time andvoltage. Typically two gas supplies are available. A typical fluorinesource 34 for a KrF laser is 1% F₂, 1% Kr and 98% Ne. A buffer gassource 36 of 1% Kr and 99% Ne may also be tapped by controller 26through valve 38 when providing an initial or a refill of the chamber orif the F₂ concentrations for some reason gets too high. Laser gas may beexhausted through valve 40 and the chamber may be drawn down to a vacuumby vacuum pump 42. Exhaust gas is cleaned of F₂ by F₂ trap 44. FIG. 3shows the results of the prior art fluorine injection techniquesdiscussed above. The voltage values represent average values of controlvoltage commands and indirectly represent average values of chargingvoltage. Since contamination gradually builds up in the laser gas over aperiod of several days, it is usually desirable to replace substantiallyall of the gas in the laser with new laser gas at intervals of about5-10 days.

Problems and Proposed Solutions

[0009] The above-described prior art technique is effectively used todayto provide long term reliable operation of these excimer lasers in amanufacturing environment. However, several laser parameters, such asbandwidth, beam profile and wavelength stability, are adversely affectedby the substantial swings in the discharge voltage and fluorineconcentration.

[0010] A substantial number of techniques have been proposed andpatented for measuring and controlling the fluorine concentration inexcimer lasers to within more narrow limits than those provided underthe above described prior art technique. These techniques have generallynot been commercially pursued. Prior art commercial excimer laserstypically do not have a fluorine monitor. A need for a good,inexpensive, reliable, real-time fluorine monitor has been recognizedfor a long time.

[0011] Techniques for measuring trace gas concentrations with lightbeams are well known. One such technique uses a photo detector todetermine the absorption of a beam as it passes through an absorptioncell. Another technique well known since it was first discovered byAlexander Graham Bell involves the creation of sound waves in anabsorption cell with an intensely modulated light beam. See OptimalOptoacoustic Detector Design, Lars-Goran Rosengren, Applied Optics Vol.14, No. 8/August 1975 and Brewsters Window and Windowless ResonanceSpectrophones for Intercavity Operations, R. Gerlach and N. M. Amer,Appl. Phys. 23, 319-326 (1980).

Injection Seeding

[0012] A well-known technique for reducing the bandwidth of gasdischarge laser systems (including excimer laser systems) involves theinjection of a narrow band “seed” beam into a gain medium. In some ofthese systems a laser producing the seed beam called a “masteroscillator” is designed to provide a very narrow bandwidth beam in afirst gain medium, and that beam is used as a seed beam in a second gainmedium. If the second gain medium functions as a power amplifier, thesystem is referred to as a master oscillator, power amplifier (MOPA)system. If the second gain medium itself has a resonance cavity (inwhich laser oscillations take place), the system is referred to as aninjection seeded oscillator (ISO) system or a master oscillator, poweroscillator (MOPO) system in which case the seed laser is called themaster oscillator and the downstream system is called the poweroscillator. Laser systems comprised of two separate systems tend to besubstantially more expensive, larger and more complicated to build andoperate than comparable single chamber laser systems. Therefore,commercial application of these two chamber laser systems has beenlimited.

[0013] What is needed is a better control system for a pulse gasdischarge laser for operation at repetition rates in the range of about4,000 to 6,000 pulses per second or greater.

SUMMARY OF THE INVENTION

[0014] The present invention provides a control system includingautomatic laser gas control, for a modular high repetition rate twodischarge chamber ultraviolet gas discharge laser. The laser gas controlincludes techniques, monitors, and processor for monitoring the F₂consumption rates through the operating life of the laser system. Theseconsumption rates are used by a processor programmed with an algorithmto determine when F₂ is to be injected to maintain laser beam qualitywithin a delivery range.

[0015] Excimer laser output performance metrics—ΔE/ΔV and bandwidth inparticular—are a function of the gas mix in both the MO and PA chambers.The optimal gas mix—the relative partial pressures of Argon or Krypton(Ar/Kr), Fluorine (F₂), Neon (Ne), etc.—is determined by laserscientists in order to meet target performance specifications. When anexcimer laser fires, F₂ is consumed through chemical reactions with theelectrodes, chamber walls and other internal components, changing the F₂partial pressure in the gas. As F₂ is consumed, it is necessary tooccasionally inject more F₂ into the chamber in order to maintain F₂concentration within an acceptable band. This provides techniques todetermine F₂ injections such that the gas mix is appropriate to enableall output performance targets to be met by the laser. Pursuant to thistechnique F₂ concentration is not measured directly; however it isreferred from other measured quantities, primarily energy output andapplied voltage. The advantage of this invention as compared to priorart techniques is that it provides a more stable, robust estimate of F₂consumption for each chamber on each laser. A more accurate estimate ofF₂ consumption rates allows the laser to be operated over a moreaccurately controlled band of F₂ concentrations, which results in bettercontrol over critical laser output parameters. F₂ consumptionrates—which slowly change as a chamber ages—are adaptively tracked overthe life of each chamber, such that the same performance (with regard toF₂ regulation) is delivered on shot 1 as on shot 10,000,000,000.

[0016] The invention makes use of an assumed equilibrium relationship inthe laser between F₂ depletion and F₂ addition. That is, the outputenergy change per voltage change (ΔE/) efficiency drop (E per V) due toF₂ depletion should be the same as the ΔE/ΔV due to F₂ addition if thesame amount of F₂ was depleted by firing the laser as was added by an F₂injection. By monitoring ΔE/ΔV decrease during operation and ΔE/ΔV riseacross each injection, which are easy to measure, and accurately knowingthe size of the injection, we can infer the amount of F₂ that wasconsumed.

[0017] There are only three basic cases to consider:

[0018] 1. ΔE/ΔV drop equals efficiency rise→F₂ consumed equals F₂ added.

[0019] 2. ΔE/ΔV drop greater than efficiency rise→More F₂ was consumedthan F₂ added.

[0020] 3. ΔE/ΔV drop less than efficiency rise→Less F₂ was consumed thanF₂ added.

[0021] Each one of these three cases implies that the true F₂consumption rate of the laser is either equal too, greater than, or lessthan, respectively, the estimate from the previous cycle. From this, anew consumption rate estimate can be adjusted accordingly. The algorithmwill eventually converge on the true consumption rate of the laser byadaptively driving toward satisfying the equilibrium relationship. For atwo-chambered laser, the efficiency of each chamber must be tracked andadjusted independently, which can be done in a similar fashion.

[0022] One distinct advantage of this algorithm is that each injectiondecision is based on the long term Active Consumption Rate Estimate ofeach chamber—which is a filtered signal that is very stable and slowlyvarying—and not the relative efficiency of that particular injectioncycle, a measurement which is subject to substantial measurement noiseand operating point changes.

[0023] This algorithm of the present invention relies on the sameunderlying physical principle as the Slope Seeking Inject (SSI)algorithm that was previously developed by Applicants' and theirco-workers and described in U.S. Pat. No. 6,151,349. However, theinformation provided by monitoring voltage rise at fixed energy outputas a function of F₂ depletion is used in a different manner. The ratioof drecrease in ΔΣ/ΔV due to F₂ depletion and the increase in ΔΣ/ΔV dueto F₂ injection are used in this new algorithm to adaptively determineF₂ consumption rate.

[0024] In a preferred embodiment, which performs very well in a twochamber MOPA system, the output energy is maintained at decreased levelsby regulating the F₂ concentration in the MO chamber. F₂ injections aredetermined by monitoring the MO output energy in relationship to the MOdischarge voltage (which in these preferred embodiments) is the same asthe PA discharge voltage. From these data needced ΔE/ΔV for the MO areobtained.

[0025] In preferred embodiments, the laser is a production line machinewith a master oscillator producing a very narrow band seed beam which isamplified in the second discharge chamber. Another important feature ofthis laser system is a feedback timing control techniques are providedfor controlling the relative timing of the discharges in the twochambers with an accuracy in the range of about 2 to 5 billionths of arecord even a burst mode operation. This MOPA system is capable ofoutput pulse energies approximately double the comparable single chamberlaser system with greatly improved beam quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 shows the typical relationship between discharge voltagefluorine concentration and pulse energy.

[0027]FIG. 2 is a schematic drawing of a prior art excimer laser system.

[0028]FIG. 3 shows a prior art graphical display of fluorineconcentration as a function of pulse count of an operating laser.

[0029]FIG. 4 is a block diagram of a MOPA Laser System.

[0030]FIG. 4A is a cutaway drawing of the FIG. 1 System.

[0031]FIG. 4B is a drawing showing a mounting technique for lasercomponents.

[0032]FIG. 4C is a block diagram showing a MOPA control system.

[0033]FIG. 4D is a block diagram of a portion of the control system.

[0034]FIG. 5 is a cross-section drawing of a laser chamber.

[0035]FIG. 6 is a schematic drawing showing features of a narrow bandlaser oscillator.

[0036]FIG. 6A is a drawing showing control features of a line narrowingunit.

[0037]FIG. 7 is a block diagram showing features of a pulse powercontrol technique.

[0038]FIG. 7A shows the results of a trigger control technique.

[0039]FIG. 8A shows relationships among E, F₂ and V.

[0040]FIG. 8B shows graphs of shot count and Voltage.

[0041] FIGS. 8C-8F show features of a first F₂ control algorithm.

[0042] FIGS. 9A-9F show features of a second F2 control algorithm.

[0043]FIG. 10A show a technique for spectrally monitoring fluorinelevels.

[0044]FIGS. 10B, 10C, and 10D show spectral results.

[0045]FIGS. 11A and 11B show data supporting an F₂ injection techniquefor a MO for an F₂ MOPA system.

[0046]FIGS. 12A, 12B and 12C show a technique for monitoring andcontrolling MO F₂ concentration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Mopa Laser LithographyLight Source General Description

[0047] A laser system incorporating a first preferred embodiment of thepresent invention is shown in FIG. 4. In this embodiment a 193 nmultraviolet laser beam is provided at the input port of a stepperlithography machine 2 such as the one of those supplied by Canon orNikon with facilities in Japan or ASML with facilities in theNetherlands. This laser system includes a laser energy control systemfor controlling both pulse energy and accumulated dose energy output ofthe system at pulse repetition rates of 4,000H_(z) or greater. Thesystem provides extremely accurate triggering of the discharges in thetwo laser chambers relative to each other with both feedback andfeed-forward control of the pulse and dose energy.

[0048] In this case the main components of the laser system 4 areinstalled below the deck on which the scanner is installed. However,this laser system includes a beam delivery unit 6, which provides anenclosed beam path for delivering the laser beam to the input port ofscanner 2. This particular light source system includes a masteroscillator 10 and a power amplifier 12 and is a type of laser systemknown as MOPA system. The light source also includes a pulse stretcher.This light source represents an important advancement in integratedcircuit light sources over the prior art technique of using a singlelaser oscillator to provide the laser light.

[0049] The master oscillator and the power amplifier each comprise adischarge chamber similar to the discharge chamber of prior art singlechamber lithography laser systems. These chambers (described in detailbelow) contain two elongated electrodes, a laser gas, a tangential forcirculating the gas between the electrodes and water-cooled finned heatexchangers. The master oscillator produces a first laser beam 14A whichis amplified by two passes through the power amplifier to produce laserbeam 14B as shown in FIG. 4. The master oscillator 10 comprises aresonant cavity formed by output coupler 10C and line narrowing package10B both of which are described generally in the background section andin more detail below in the referenced patents and parent applications.The gain medium for master oscillator 10 is produced between two 50-cmlong electrodes contained within master oscillator discharge chamber10A. Power amplifier 12 is basically a discharge chamber and in thispreferred embodiment is almost exactly the same as the master oscillatordischarge chamber 10A providing a gain medium between two elongatedelectrodes but power amplifier 12 has no resonant cavity. This MOPAconfiguration permits the master the master oscillator to be designedand operated to maximize beam quality parameters such as wavelengthstability and very narrow bandwidth; whereas the power amplifier isdesigned and operated to maximize power output. For example, the currentstate of the art light source available from Cymer, Inc. Applicants'employer, is a single chamber 5 mJ per pulse, 4 kHz, ArF laser system.The system shown in FIG. 4 is a 10 mJ per pulse (or more, if desired) 4kHz ArF laser system producing at least twice the average ultravioletpower with substantial improvement in beam quality. For this reason theMOPA system represents a much higher quality and much higher power laserlight source. FIG. 4A shows the general location of the above referredto components in one version of the MOPA modular laser system.

The Master Oscillator

[0050] The master oscillator 10 shown in FIGS. 4 and 4A is in many wayssimilar to prior art ArF lasers such as described in the '884 patent andin U.S. Pat. No. 6,128,323 and has many of the features of the ArF laserdescribed in U.S. patent application Ser. No. 09/854,097 except theoutput pulse energy is typically about 0.1 mJ instead of about 5 mJ. Asdescribed in great detail in the '097 application, major improvementsover the '323 laser are provided to permit operation at 4000 Hz andgreater. The master oscillator of the present invention is optimized forspectral performance including precise wavelengths and bandwidthcontrol. This result is a much more narrow bandwidth and improvedwavelength stability and bandwidth stability. The master oscillatorcomprises discharge chamber 10A as shown in FIG. 4, FIG. 4A, and FIG. 5in which are located a pair of elongated electrodes 10A2 and 10A4, eachabout 50 cm long and spaced apart by about 0.5 inch. Anode 10A4 ismounted on flow shaping anode support bar 10A6. Four separate finnedwater-cooled heat exchanger units 10A8 are provided. A tangential fan10A10 is driven by two motors (not shown) for providing a laser gas flowat velocities of up to about 80 m/s between the electrodes. The chamberincludes window units (not shown) with CaF₂ windows positioned at about45° (or about 70°) with the laser beam. An electrostatic filter unithaving an intake at the center of the chamber, filters a small portionof the gas flow as indicated at 11 in FIG. 5 and the cleaned gas isdirected into each of the window units in the manner described in U.S.Pat. No. 5,359,620 (incorporated herein by reference) to keep dischargedebris away from the windows. The gain region of the master oscillatoris created by discharges between the electrodes through the laser gaswhich in this embodiment is comprised of about 3% argon, 0.1% F₂ and therest neon. The gas flow clears the debris of each discharge from thedischarge region prior to the next pulse. The resonant cavity is createdat the output side of the oscillator by an output coupler 10C (as shownin FIG. 4) which is comprised of a CaF₂ mirror mounted perpendicular tothe beam direction and coated to reflect about 30% of light at 193 nmand to pass about 70% of the 193 nm light. The opposite boundary of theresonant cavity is a line narrowing unit 10B as shown in FIG. 4 similarto prior art line narrowing units described in U.S. Pat. No. 6,128,323.Important improvements in this line narrowing package as shown in FIG. 5include four CaF beam expanding prisms 112 a-d for expanding the beam inthe horizontal direction by 45 times and a tuning mirror 114 controlledby a stepper motor for relatively large pivots and a piezoelectricdriver for providing extremely fine tuning of the center linewavelength. FIG. 6A shows the stepper motor 82 and piezoelectric driver83. The stepper motor provides its force to mirror 114 through lever arm84 and piezoelectric driver 83 applies its force on the fulcrum 85 ofthe lever system. An LNP processor 89 located at the LNP controls boththe stepper motor and the piezoelectric driver based on feedbackinstructions from a line center analysis module (LAM) 7. Echelle grating10C3 having about 80 facets per mm is mounted in the Litrowconfiguration and reflects a very narrow band of UV light selected fromthe approximately 300 pm wide ArF natural spectrum. Preferably themaster oscillator is operated at a much lower F₂ concentration than istypically used in prior art lithography light sources. This results insubstantial reductions in the bandwidth since Applicants have shown thatbandwidth decreases substantively with decreasing F₂ concentrations.Another important improvement is a narrow rear aperture which limits thecross section of the oscillator beam to 1.1 mm in the horizontaldirection and 7 mm in the vertical direction. Control of the masteroscillator beam is discussed below.

[0051] In preferred embodiments the main charging capacitor banks forboth the master oscillator and the power amplifier are charged inparallel so as to reduce jitter problems. This is desirable because thetimes for pulse compression in the pulse compression circuits of each ofthe two pulse power systems is very dependent on the level of the chargeof the charging capacitors. Preferably pulse energy output is controlledon a pulse-to-pulse basis by adjustment of the charging voltage. Thislimits the use of voltage to control beam parameters of the masteroscillator. However, laser gas pressure and F₂ concentration can beeasily controlled separately in each of the two chambers to achievedesirable beam parameters over a wide range of pulse energy levels andlaser gas pressures. Bandwidth decreases with decreasing F₂concentration and laser gas pressure. These control features are inaddition to the LNP controls which are discussed in detail below.

Power Amplifier

[0052] The power amplifier in this preferred embodiment is comprised ofa laser chamber which, with its internal components, as stated above isvery similar to the corresponding master oscillator discharge chamber.Having the two separate chambers allows the pulse energy and dose energy(i.e., integrated energy in a series of pulses) to be controlled, to alarge extent, separately from wavelength and bandwidth. This permitshigher power and better dose stability. All of the components of thechamber are the same and are interchangeable during the manufacturingprocess. However, in operation, the gas pressure is substantially higherin the PA as compared to the MO since laser efficiency increases with F₂concentration and laser gas pressure over wide ranges. The compressionhead 12B of the power amplifier is also substantially identical in thisembodiment to the 10B compression head of the MO and the components ofthe compression heads are also interchangeable during manufacture. Thisclose identity of the chambers and the electrical components of thepulse power systems helps assure that the timing characteristics of thepulse forming circuits are the same or substantially the same so thatjitter problems are minimized. One minor difference is that thecapacitors of the MO compression head capacitor bank are more widelypositioned to produce a substantially higher inductance as compared tothe PA.

[0053] The power amplifier is configured for two beam passages throughthe discharge region of the power amplifier discharge chamber as shownin FIG. 4. The beam oscillates several times through the chamber 10Abetween LNP 10B and output coupler 10C (with 30 percent reflectance) ofthe MO 10 as shown in FIG. 1 and is severely line narrowed on itspassages through LNP 10C. The line narrowed seed beam is reflecteddownward by a mirror in the MO wavelength engineering box (MO WEB) 24and reflected horizontally at an angle slightly skewed (with respect tothe electrodes orientation) through chamber 12. At the back end of thepower amplifier beam reverser 28 reflects the beam back for a secondpass through PA chamber 12 horizontally in line with the electrodesorientation.

[0054] The charging voltages preferably are selected on a pulse-to-pulsebasis to maintain desired pulse and dose energies. F₂ concentration andlaser gas pressure can be adjusted to provide a desired operating rangeof charging voltage (since as indicated above charging voltage decreaseswith increasing F₂ concentration and gas pressure for a given outputpulse energy). This desired range can be selected to produce a desiredvalue of dE/dV since the change in energy with voltage is also afunction of F₂ concentration and laser gas pressure. F₂ gas is depletedin the chambers over time and the depletion is in general accommodatedby a corresponding increase in charging voltage to maintain desiredpulse energy. Detailed descriptions of preferred injection techniquesare described below. The frequency of injections preferably is kept high(and the inserted quantity is preferably kept small) to keep conditionsrelatively constant and injections can be continuous or nearlycontinuous. However, some users of these laser systems may prefer largerdurations (such as 2 hours) between F₂ injections. Some users may preferthat the laser be programmed to not fire during F₂ injections.

MOPA Control System

[0055]FIG. 4C is a block diagram showing many of the important controlfeatures of a preferred embodiment of the present invention. The controlsystem includes RS232 laser scanner interface hardware 600 whichcomprises special software permitting laser control from any of severaltypes of lithography machines 2 (which could be a stepper or scannermachine) or a laser operation control paddle 602. Central processingunit 604 is the master control for the MOPA system and receivesinstructions through four serial ports 606 and interface hardware 600,from lithography machine 2 and operator control paddle 602.

[0056] Laser control CPU 604 communicates to fire control CPU 608through communication CPI bases 610, 612, and 614. Fire control platformCPU 608 controls the charging of the charging capacitors in both the MOand the PA which are charged in parallel by resonant charger 49. Firecontrol CPU 608 sets the HV target for each pulse and provides thetrigger to begin charging. (This CPU also implements timing control andenergy control algorithms discussed in more detail below). A timingenergy module 618 receives signals from light detectors in MO and PAphoto detector modules 620 and 622 and based on these signals andinstructions from command module 616 provides feedback trigger signalsto commutator 50A and PA commutator 50B which triggers switchesinitiating discharges from the MO and PA charging capacitors 42 as shownin FIG. 5 and initiates the pulse compressions resulting in thegeneration of discharge voltage in the peaking capacitors 82 to producedischarges in the MO and the PA. Additional details of the TEM are shownin FIG. 4D.

[0057] The preferred timing process is as follows: command module 616sends trigger instructions to timing energy module 618 27 microsecondsprior to desired light providing the precise times for triggeringswitches 46 in both the MO and the PA. The TEM synchronizes timingsignals with its internal clock by establishing a reference time calledthe “TEM reference” and then correlates trigger and light out signals tothat reference time. The TEM then issues trigger signals to MO switch 46in the MO commutator 50A with an accuracy of about 25 picoseconds andabout 30 to 50 ns later (in accordance with the instructions fromcommand module 616) issues a trigger signal to the PA switch 46 in thePA commutator 50B also with an accuracy of about 25 ps. The TEM thenmonitors the time of light out signals from PD modules 620 and 622 withan accuracy better than 100 ps relative to the TEM reference time. Thesetime data are then transmitted by the TEM 618 to command module 616which analyzes these data and calculates the proper timing (based onalgorithms discussed below) for the next pulse and 27 microseconds priorto the next pulse, command module 616 sends new trigger instructions totiming energy module 618.

[0058] Thus, the discharge timing job is shared between TEM module 618and command module 616. Communication between the two modules is along10 megabit synchronous serial link shown at 617 in FIG. 1C. Module 618provide extremely fast trigger generation and timing methodology andmodule 616 provides extremely fast calculations. Both working togetherare able to monitor timing, provide feedback, calculate the next timingsignal using a complicated algorithm and provide two trigger signals tothe commutators all within time windows of less than 250 microsecondsand to assure relative triggering accuracy of the two discharges of lessthan about 2 to 5 billions of a second! TEM module also provide a lightout signal to stepper/scanner 2. This triggering process can be modifiedby instructions from the stepper/scanner 2 or by the laser operatorthrough user interface paddle 602. High speed monitoring and triggercircuits of the type used in TEM module are available from supplierssuch as Highland Technologies with offices in San Francisco, San Rafaeland Berkley, California. The importance of the accuracy of these timingcircuits and issues and features relating to these trigger circuits arediscussed in more detail below.

[0059] Wavelength control is provided by LNM controller 624 withinstructions from fire control platform 608 based on feedback signalsfrom line center analysis module 7 which monitors the output of the MO.Preferred techniques for measuring the line center are discussed below.

[0060] Control of other elements of the laser system is provided by acontrol area network (CAN) as indicated on FIG. 4C. CAN interface 626interfaces with laser control platform 604 and provides controlinformation to three CAN clusters: power cluster 628, left optics baycluster 630, and right optics bay cluster 632. This CAN network providestwo-way communication with these modules providing control from lasercontrol platform 604 to the various modules and providing operationaldata from the modules back to the laser control platform.

[0061] Data acquirization can be provided through switch 636Cymer-on-Line module 634 which can collect and store high amounts ofdata and make it available through Internet systems all as described inU.S. patent application Ser. No. 09/733,194, which is incorporated byreference herein. Field services port 638 provides access to CPU 608 andCPU 604 for special analysis and tests. Also eight BNC connectors 640are available through digital-to-analog converter 642 for specialmonitors.

Pulse Power Circuit

[0062] In the preferred embodiment shown in FIG. 4 the basic pulse powercircuits for both the MO and the PA are similar to pulse power circuitsof prior art excimer laser light sources for lithography. Separate pulsepower circuits downstream of the charging capacitors are provided foreach discharge chamber. Preferably a single resonant charger charges twocharging capacitor banks connected in parallel to assure that bothcharging capacitor banks are charged to precisely the same voltage. Thispreferred configuration is shown in FIG. 7. Details of the pulse powersystem are described in U.S. patent application Ser. No. 10/210,761filed Jul. 31, 2002 which is incorporated by reference herein.

Pulse and Dose Energy Control

[0063] Pulse energy and dose energy are preferably controlled with afeedback control system and algorithm such as that described in U.S.Pat. No. 6,067,306 which is incorporated herein by reference. The pulseenergy monitor can be at the laser or closer to the wafer in thelithography tool. As described above the Co charging capacitors of boththe PA and the MO are charged in parallel to the same voltage. Thecharging voltages are chosen to produce the pulse energy desired.Applicants have determined that this technique works very well andgreatly minimize timing jitter problems. This technique, however, doesreduce to an extent the laser operator's ability to control the MOindependently of the PA. However, there are a number of operatingparameters of the MO and the PA that can be controlled separably tooptimize performance of each unit. These other parameters include: lasergas pressure, F₂ concentration and laser gas temperature. Theseparameters preferably are controlled independently in each of the twochambers and regulated in a processor controlled feedback arrangement.

Gas Control

[0064] A preferred embodiment of this invention has a gas control modulelabeled “Gas” in FIG. 4A and it is configured to fill each chamber withappropriate quantities of laser gas. Preferably appropriate controls andprocessor equipment is provided to maintain continuous flow of gas intoeach chamber so as to maintain laser gas concentrations constant orapproximately constant at desired levels. This may be accomplished usingtechniques such as those described in U.S. Pat. No. 6,028,880 or U.S.Pat. No. 6,151,349 or U.S. Pat. No. 6,240,117 (each of which areincorporated hereby reference). In one embodiment about 3 kP of fluorinegas (comprised of, for example, 1.0% F₂, 3.5% Ar and the rest neon forthe ArF laser) is added to each chamber each 10 million pulses. (At 4000Hz continuous operation this would correspond to an injection eachapproximately 42 minutes and at a 20 percent duty factor injectionswould be every 3.5 hours.) Periodically, the laser is shut down and thegas in each chamber is evacuated and the chambers are refilled withfresh gas. Typical refills are at about 100,000,000 pulses for ArF andabout 300,000,000 for KrF.

[0065] A technique for providing substantially continuous flow of lasergas into the chambers which Applicants call its binary fill technique isto provide a number (such as 5) fill lines each successive line orificedto permit double the flow of the previous line with each line having ashut off valve. The lowest flow line is orificed to permit minimumequilibrium gas flow. Almost any desired flow rate can be achieved byselecting appropriate combinations of valves to be opened. Preferably abuffer tank is provided between the orificed lines and the laser gassource which is maintained at a pressure at about twice the pressure ofthe laser chambers.

[0066] Gas injections can also be automatically made when chargingvoltage levels reach predetermined values. These predetermined levelsmay be established by performance of the laser efficiency tests in whichlaser efficiency is measured at a variety of values of F₂ concentrationand total gas pressure. The efficiency tests may also be performed inthe course of gas refills after the laser has become part of aintegrated circuit production line. Initial values for F₂ concentrationand total gas pressure are preferably different for the MO and the PA.The levels of F₂ concentration and total gas pressure for the MO arechosen for best band width MO energy output and pulse energy stabilityand the corresponding values for the PA are chosen for best pulse energystability and desired laser efficiency. As indicated below laser gasconditions may sometimes be set at values to purposely reduce the laserefficiency. For example, operation at charging voltages substantiallybelow design value often results in degraded beam qualities such aspulse energy stability. Sometime it may be desirable to trade-off someefficiencies for better pulse energy stability or other beam parameters.

First Preferred F₂ Inject Algorithm Theory of Operations

[0067] F₂ Consumption

[0068] A preferred F₂ inject algorithm for the MOPA power amplifierchambers 12 as shown in FIG. 4 can be described by reference to FIGS. 8Athrough 8F. This algorithm may also be used for F₂ control in chamber10. This algorithm is based on the assumption that F₂ is consumed at an“active rate” where the consumption is based on the number of pulses anda “passive rate” where the consumption is based on the passage of time.If these two rates were exactly known, then knowledge of the number ofpulses and the passage of time would precisely determine the amount ofF₂ needed to be injected to maintain F₂ concentration at any desiredvalues.

[0069] Since the F₂ consumption rates (active or passive) are not knownexactly for any given chamber, it would be insufficient to simply setfixed active and passive consumption rates and rely on counting shotsand time. The algorithm outlined below provides a mechanism toadaptively determine the active F₂ consumption rate for each laserthroughout its operating life, and to use this value in combination withpulse count to continuously estimate F₂ consumption. A target amount ofF₂ to be consumed is set at the start of each cycle, and an injection isperformed when the consumption estimate crosses this threshold. Due tothe relatively small contribution of passive F₂ consumption to totalconsumption, a fixed value for passive F₂ consumption rate will be usedon all lasers.

[0070] Key Assumptions

[0071] F₂ consumption rate is independent of operating condition (reprate, voltage, duty cycle, etc.)

[0072] F₂ consumption rate varies slowly over the life of a chamber

[0073] [F₂] can be inferred from some repeatable, measurable phenomenonin the system

[0074] F₂ concentration (sometimes designated as [F₂])

[0075] [F₂] is known accurately at the time of a gas refill

[0076] Change in [F₂] due to an inject is accurately known

[0077] Injection Decision Mechanism

[0078] A primary design goal of this algorithm was to reduce the F₂injection decision to a single, unambiguous criterion that would beinsensitive to laser operating conditions and measurement noise. To thisend, three values are fixed at the start of each F₂ consumption cycle(that is, just after either a refill or an inject): amount of F₂ to beconsumed during the cycle [kPa], estimated active consumption rate (ACRin [kPa]/Mshot), and passive consumption rate (PCR in [kPa]/hour).Typical numbers for each of these three values would be 3.0 kPa, 0.3kPa/Mshot, and 0.05 kPa/hour. (The reader should note as explained abovethat 3.0 kPa is partial pressure value representing the partial pressureof F₂ gas mix of 1% F₂, 3% argon and 96% neon. For a 301 chamber thisrepresents about 0.02 grams of fluorine gas).

[0079] As the laser is fired, a running estimate of the amount of F₂that has been consumed in the current cycle is computed from shot countand elapsed time: $\begin{matrix}{\left\lbrack F_{2} \right\rbrack_{consumed} = {{\frac{\left( {{shots}\quad {since}\quad {inject}} \right)}{1,000,000} \times \left( {{ACR}\frac{\lbrack{kPa}\rbrack}{Mshot}} \right)} + {\left( {{hours}\quad {since}\quad {inject}} \right) \times \left( {{PCR}\frac{\lbrack{kPa}\rbrack}{hour}} \right)}}} & \lbrack 1\rbrack\end{matrix}$

[0080] This value is compared to the target amount of F₂ to consumeduring the current cycle, and when the target amount has been consumed,an injection is requested:

if ([F₂]_(consumed)>[F₂]_(target))

[0081] do injection;

[0082] No other logic enters into the F₂ injection decision.

[0083] Active Consumption Rate Estimation (ACRE)

[0084] The interesting part of this algorithm comes in the estimation ofthe active consumption rate. This is accomplished through a combinationof a time-tested Slope Seeking Inject (SSI) algorithm and a newalgorithmic addition dubbed “Continuity” that makes SSI more robust tooperating point changes.

[0085] Slope Seeking Inject

[0086] The Slope Seeking Inject algorithm is based on the recognitionthat Σ/V (output energy at a given input voltage) is a function of [F₂],as well as several other operating point parameters. Assuming for amoment that the operating point of the laser is fixed (where “operatingpoint” here refers to a specific combination of target energy, rep rate,burst length and duty cycle), the voltage required to deliver a giventarget energy will rise as F₂ is consumed. This effect is illustrated inFIG. 8A which depicts experimental data for a power amplifier chamber 12as shown in FIG. 4.

[0087] The traditional SSI algorithm functions by monitoring the voltagerise due to F₂ consumption and performing an F₂ injection when thevoltage rise exceeds some threshold value. The threshold value isadjusted between each consumption cycle such that the initial voltage isrecovered following an injection, with the assumption that this voltagealso corresponds to the initial [F₂] for a particular output energy.Because the relationship between [F₂] and ΔΣ/ΔV is nonlinear, it takesmultiple injections to reach a self-consistent voltage rise target thatrecovers the initial voltage. Simulation data showing two consumptioncycles with one injection at 10M shots is shown in FIG. 8B.

[0088] The SSI method can break down in the presence of operating pointchanges, which affect not only the current ΔΣ/ΔV value but also thereference ΔΣ/ΔV value. Energy target changes can be handled reasonablywell by making use an estimate of ΔΣ/ΔV to adjust the reference voltage,but rep rate and duty cycle changes are known to confuse prior art SSIalgorithms. This issue will be addressed later, in the “Continuity”section.

[0089] This algorithm makes use of the basic SSI concept, but in a waywhich is different from the prior art. Rather than relying on thevoltage rise due to F₂ consumption to determine injection times, thealgorithm monitors the voltage drop across each injection to determinethe relationship between [F₂] and laser efficiency. Because theinjection amount is known and the injection time is short, this methodprovides a much more accurate number on which to base consumptionestimates. Voltage values used for this calculation are taken whenoperating conditions such as repetition rate and output pulse energy arethe same (before and after the injection) The short time scale reducesthe likelihood that other laser operating conditions would have changedsignificantly, introducing errors in the voltage change measurement. Asmall amount of rare gas mixtures can be injected to force the F₂ gasremaining in the gas line between the gas manifold and the chamber tomove into the chamber in order to ensure a fast response of the chamberoperating voltage to an F₂ injection.

[0090] All voltage changes are based on a quantity dubbed the burstaverage voltage (BAV), which as the name implies is just the averagevoltage of every pulse in a burst, including the initial transient andreentrant slug. This definition is preferred here over the 10,000 pulsemoving average because it is easier to observe changes in BAV acrossburst boundaries when an operating point change is made. Since operatingpoints do not change mid-burst, the burst average is the naturalgranularity for this algorithm. The BAV is further filtered through anN-burst moving window, where N is configurable but will likely be anorder of about 20 bursts (O[20]) for good algorithm performance.

[0091] Returning to algorithm, the voltage drop across an inject incomputed by storing the BAV before starting the inject, performing theinject, then allowing the BAV to settle to a new equilibrium:

ΔV _(inject) =BAV _(pre-inject) −BAV _(post-inject)

[0092] Consumption Sensitivity Factor

[0093] The prescribed injection size and measured voltage drop are usedto compute a consumption sensitivity factor, in units of [kPa]/V, asfollows: $\begin{matrix}{\left\lbrack F_{2} \right\rbrack_{\Delta \quad V} = {\frac{\left\lbrack F_{2} \right\rbrack_{{inject}{size}}}{\Delta \quad V_{inject}}\quad {or}\quad \frac{\left\lbrack F_{2} \right\rbrack_{{inject}{size}}}{\sqrt{\Delta \quad V_{inject}}}}} & \lbrack 4\rbrack\end{matrix}$

[0094] Two forms of this scale factor are provided above, with thesecond taking the square root of voltage drop. Recognizing that therelationship between voltage and [F₂] has been observed to beapproximately quadratic, it is believed that the second form above willprovide better numerical behavior in subsequent linear correction steps,but this has not been verified yet in practice, and will be left as anopen issue.

[0095] The response of a given laser to a fixed size inject should befairly constant and repeatable. As each inject is performed, the mostrecent consumption sensitivity value, [F₂]_(ΔV) ^(last), is used toupdate the running value for the system: $\begin{matrix}{\left\lbrack F_{2} \right\rbrack_{\Delta \quad V} = {\left\lbrack F_{2} \right\rbrack_{\Delta \quad V} + {k \times \left( {\left\lbrack F_{2} \right\rbrack_{\Delta \quad V}^{last} - \left\lbrack F_{2} \right\rbrack_{\Delta \quad V}} \right)}}} & \lbrack 5\rbrack\end{matrix}$

[0096] This simply provides low pass filtering of the measurement toreduce the impact of noise from one inject to the next. In this way, theconsumption sensitivity factor is used adaptively estimate and track ACRof the laser over its lifetime, and is maintained as a persistentparameter in the control system.

[0097] Consumption Estimation

[0098] As outlined above, the consumption sensitivity factor isdetermined from the voltage drop across injects rather than the voltagerise during a consumption cycle. It is assumed, however, that this samescale factor applies in both directions. By tracking the voltage risesince an inject, ΔV_(rise), and applying the consumption scale factor tothis voltage (or square-root of voltage, TBD), an estimate is given ofthe approximate quantity of F₂ that has been consumed:

[F ₂]_(consumed) ≅[F ₂]_(ΔV) ×ΔV _(rise)

[0099] Combining equations [4] ανδ [6], and ignoring momentarily thelow-pass filtering of Eq [5], the simple relationship between voltageand [F₂] becomes clear: $\begin{matrix}{\left\lbrack F_{2} \right\rbrack_{consumed} = {\left\lbrack F_{2} \right\rbrack_{{inject}{size}} \times \frac{\Delta \quad V_{rise}}{\Delta \quad V_{inject}}}} & \lbrack 7\rbrack\end{matrix}$

[0100] If the voltage rise with consumption and voltage drop withinjection are exactly equal, the implication is that the quantity of F₂consumed was exactly equal to the mount injected. The existing SSIalgorithm implicitly seeks this equilibrium by adjusting the targetΔV_(rise) that results in a return to the initial reference voltage.

[0101] Active Consumption Rate

[0102] In practice, relying on Eq. [6] and the filtered estimate of[F₂]_(ΔV) from Eq. [5] should yield a fairly accurate estimate of theactual amount of F₂ consumed during a given cycle. From this, a newactive consumption rate estimate can be made by dividing the consumptionquantity by the actual number of shots fired on the last cycle:$\begin{matrix}{{ACR}^{last} = \frac{\left\lbrack F_{2} \right\rbrack_{consumed}^{last}}{{\left( {{shots}\quad {at}\quad {inject}} \right)/\quad 1},000,000}} & \lbrack 8\rbrack\end{matrix}$

[0103] Of course, this value will be low-pass filtered to update therunning value for ACR:

ACR=ACR+k×(ACR ^(last) −ACR)

[0104] This value should converge within the first gas life of a laserchamber to a value that varies slowly over the entire life of the laser.

[0105] Consumption Target

[0106] Recall that each injection decision is based purely on shot countand elapsed time, as indicated in Eq. [1]. However, the target amount ofF₂ to consume in a given cycle is adjustable, as seen in Eq. [2], andthis target value need not be exactly equal to the inject size. Sincethe ACRE algorithm is designed to accurately estimate the quantity of F₂consumed in a given cycle, it is a simple matter to estimate the F₂remainder at the end of that cycle: $\begin{matrix}{\left\lbrack F_{2} \right\rbrack_{remainder} = {\left\lbrack F_{2} \right\rbrack_{{inject}{size}} - \left\lbrack F_{2} \right\rbrack_{consumed}}} & \lbrack 10\rbrack\end{matrix}$

[0107] Note that this “remainder” may be an excess of F₂ (for example,if the previous ACR were too high) or more likely a deficit. For certaincustomers, an F₂ injection does not necessarily occur as soon as it isrequested, resulting in more than the target amount of F₂ beingconsumed. Both the ACR and the remainder calculation are based on whenan inject actually occurred, rather than when it was requested, whichprovides a mechanism for maintaining the appropriate [F₂] in thepresence of oddly timed injects.

[0108] The consumption target for a given cycle is determined directlyfrom the configurable inject size and the remainder computation of Eq.[10]: $\begin{matrix}{\left\lbrack F_{2} \right\rbrack_{target} = {\left\lbrack F_{2} \right\rbrack_{{inject}{size}} + {c \times \left\lbrack F_{2} \right\rbrack_{remainder}}}} & \lbrack 11\rbrack\end{matrix}$

[0109] Eq. [11] employs a confidence scale factor, c, on the remainderterm of the consumption target. It was found through simulation thatblind reliance on the remainder calculation led to undesirablefluctuations in the F₂ consumption target. A simple way to establishconfidence in a given estimate is to compare the last ACR value with therunning ACR value for the system. A close match of these values wouldsuggest that the current consumption estimate is reliable, while a largedeviation would imply a lack of confidence in the estimate. Currentsimulations apply a fixed error threshold for using the remaindercalculation when setting the next consumption target, and a confidencescale factor of either 0 or 0.5.

[0110] Operating Point Changes (“Continuity”)

[0111] The preceding analysis relies on having an accurate measure ofboth the voltage rise during a consumption cycle and the voltage dropfollowing an F₂ injection. In the absence of operating point changes,voltage rise is a monotonically increasing function which is roughlyproportional to shot count squared. However, laser efficiency changeswith changes in certain operating point parameters—notably energytarget, rep rate, and duty cycle. The standard SSI algorithm wouldinterpret these changes as either F₂ addition or consumption, dependingon whether efficiency rose or fell, respectively, when in fact [F₂]should not be affected at all.

[0112] The Continuity concept takes advantage of the fact that thevoltage change due to F₂ consumption happens on a much longer time scale(hours) than the voltage change due to an operating point change(minutes). It is a straight-forward matter to detect an operating pointchange, store the pre-change burst average voltage value, wait for thelaser to settle at a new voltage, then note the post-change BAV value.The observed delta in voltage can be attributed entirely to theoperating point change due to the above time scale argument. Bymaintaining a continuous voltage rise (from the initial reference value)across this change, the SSI mechanism can be maintained.

[0113] To see how this would work in practice, begin with the basicdefinition of ΔV_(rise), which is simply the difference between therunning value of BAV and the reference voltage established at the startof the consumption cycle:

ΔV_(rise) =BAV _(current) V _(ref)

[0114] Immediately following an operating point change, the running BAVwill exhibit a step change in voltage relative to the pre-change value:

BAV _(post-change) =BAV _(pre-change) +BAV _(op-change)

[0115] A continuous voltage rise across the operating point change canbe maintained by adding the voltage step to the reference voltage:$\begin{matrix}\begin{matrix}{{\Delta \quad V_{rise}} = {{BAV}_{{pre}\text{-}{change}} - V_{ref}}} \\{= {\left( {{BAV}_{{pre}\text{-}{change}} + {\Delta \quad V_{{op}\text{-}{change}}}} \right) - \left( {V_{ref} + {\Delta \quad V_{{op}\text{-}{change}}}} \right)}} \\{= {{BAV}_{{post}\text{-}{change}} - \left( {V_{ref} + {\Delta \quad V_{{op}\text{-}{change}}}} \right)}}\end{matrix} & \lbrack 14\rbrack\end{matrix}$

[0116] Multiple operating point changes within a given consumption cyclecan be absorbed into a single voltage offset from the original referencevalue, such that: $\begin{matrix}\begin{matrix}{{\Delta \quad V_{rise}} = {{BAV} - \left( {V_{ref} + V_{offset}} \right)}} \\{V_{offset} = {\Sigma \quad \Delta \quad V_{{op}\text{-}{change}}}}\end{matrix} & \lbrack 15\rbrack\end{matrix}$

[0117] For the purpose of computing the voltage rise due to F₂consumption, a new reference voltage is established at the start of eachconsumption cycle. The voltage offset from reference is rezeroed at thesame time.

First Preferred Algorithm Specification

[0118] Purpose

[0119] The purpose of this section is to provide a top-level state chartand detailed flow charts for each state to define all of the logic toimplement the algorithm outlined in the Theory of Operation section. Allequations and symbols are (hopefully) consistent between the twosections. Where appropriate, guides will be provided as to how thealgorithm should be implemented based on existing prototype code andsimulations. FIG. 8C is a top-level state diagram, which provides asummary view of the F₂ Inject algorithm. Each state will be discussedindividually in the following sections.

[0120] Basic Operating States

[0121] For the purposes of the F₂ Inject algorithm, there are only threeoperating states for the laser: Refill/Inject, Fixed Op Point, and NewOp Point. During normal operation, the laser will spend most of it'stime in Fixed Op Point, running at some fixed operating condition whilethe F₂ Inject algorithm simply monitors the moving average of burstaverage voltage. This state will be discussed first.

[0122] Fixed Op Point

[0123] An outline of the portion of this preferred algorithm coveringthe fixed Op Point state is shown in FIG. 8D. As mentioned previously,the natural granularity for this algorithm is at the burst level, sinceoperating conditions are fixed within a burst. Between each burst, theburst average voltage must be computed from the individual high voltageset points of all the pulses in that burst. In addition, it is suggestedthat burst average energy be computed for both the MO and PA chambers.This averaging provides some filtering of the data and obviates the needto distinguish between burst transient voltage, final voltage, orvoltage target for a burst. In prototype code, burst average voltage wascomputed as a one-line addition to the other pulse-to-pulse processing,and passed to a routine that only executed between bursts. The burstaverage voltage is run through a moving average filter, which providesaddition smoothing of the data. Voltage rise is computed at the end ofeach burst so that the value is available if an op point change orinject request is issued, either manually or automatically.

[0124] Between each burst, two steps are taken to determine if an F₂injection is needed. First, the estimated amount of [F₂] consumed sincethe previous injection is computed from a combination of shot count(active consumption) and elapsed time (passive consumption). This valueis then compared with the consumption target. If more than the targetamount of F₂ has been consumed, an injection is requested. Depending onthe customer, the injection will either begin immediately (resulting ina transition to the Refill/Inject state), or the algorithm will waituntil the request for injection has been accepted. In the later case,the F₂ algorithm will continue to operate as normal in the “Fixed OpPoint” state, monitoring voltage rise and [F₂] consumption. When thesignal for Inject is given by customer hardware, the transition toRefill/Inject will occur. In this way, a correct accounting of F₂consumed and voltage rise due to consumption will be maintained.

[0125] Refill/Inject

[0126] This Refill/Inject state provides a starting point for each F₂consumption cycle. On entry to this state, an [F₂] consumption target isset for the cycle, along with a value to be used as the activeconsumption rate. Both of these values are fixed for the entire cycle.

[0127] As can be seen from the state flow chart in FIG. 8E, differentactions are taken depending on whether the state was entered following arefill or an injection request. In the case of a refill, the consumptiontarget is equal to the (fixed) injection size and the ACR is read frompersistent memory. If a consumption cycle has just completed and thisstate is entered for an injection, the consumption target is computedfrom a combination of the injection size and the estimated [F₂]remainder from the previous cycle. The voltage rise during the previouscycle is used to update the active consumption rate. This new ACR isthen used for the next cycle.

[0128] Note that two different methods are used to compute consumptionamount in this algorithm. During a consumption cycle, ACR times shotcount plus PCR times elapsed time is used to determine the amount of F₂consumed for the purpose of timing the next injection. In theRefill/Inject state, voltage rise times injection sensitivity is usedinstead. When the algorithm is converged, these values should all bemutually consistent. However, it is the job of the Inject state to findthe value of ACR that makes this true.

[0129] The actual F₂ injection occurs while in the Refill/Inject state,as indicated by the small “Do F₂ Injection” block in the state Entrycode block. At this point, refer to the detailed diagram in anotherdocument that describes how to actually perform an F₂ injection from anuts and bolts perspective—i.e., open Valve A, shoot gas in tube, closeValve A.

[0130] After the [F₂] target and ACR for the next cycle have beendetermined, the main purpose of the Refill/Inject state is to establishthe reference voltage against which voltage rise is measured. This isdone by monitoring the moving average of burst average voltage for somenumber of bursts (configurable, but probably O[50]). When the prescribednumber of bursts is exceeded, the reference voltage is set, and thestate exit code is run.

[0131] Following an inject, the exit code for this state updates theinternal estimate of F₂ consumption sensitivity, [F₂]_(ΔV). This valueis just the ratio of F₂ added to voltage drop. A simple low-pass filteris applied to the data to smooth out the measurement from one inject tothe next.

[0132] New Op Point

[0133] The New Op Point state, as described graphically in FIG. 8F,handles changes in operating point—the combination of energy set point,rep rate, burst length, and interburst interval—which can throw off theconsumption rate estimator. Because a change in any one of theseparameters may result in a step change in burst average voltage, it isnecessary to correct for this effect in order to properly track voltagerise due to F₂ depletion. The basic mechanism is the same as that usedto determine the reference voltage in the Refill/Inject state. Movingaverage of burst average voltage is tracked for a prescribed number ofbursts. When this number is exceeded, the voltage change due to theoperating point change is computed, and this change is added to thereference voltage offset.

[0134] Referring back to the top-level state transition diagram, thereis a path from Refill/Inject to New Op Point. This path will be taken ifan operating point change occurs before a reference voltage has beenestablished immediately following an injection. In that case, thecorrect behavior is to not update the consumption sensitivityfactor—which would require a converged reference voltage—and to let NewOp Point handle establishing the reference voltage for the new cycle. Ineither case, New Op Point always transitions to Fixed Op Point uponsatisfying its exit condition.

[0135] Implementation Notes Convergence Exit Criterion

[0136] In the current algorithm specification, both the Refill/Injectand New Op Point states have an exit criterion that requires a fixedburst count to establish a new burst average voltage. A more refinedexit condition check would be to monitor the gradient of burst averagevoltage following either a Refill/Injection or op point change, anddeclare the appropriate reference voltage to be established when thegradient falls below a prescribed threshold. This would ensure that thealgorithm did not spend too long in either Refill/Inject or New OpPoint, and that the reference voltage used to determine voltage rise wasnot establish while voltage was still settling.

[0137] Op Point Change Detection

[0138] A simple function can be used to determine if any of the keyoperating point parameters have changed between bursts. In order toavoid spurious detections, prototype code set percentage changethresholds that would trigger New Op Point being called. For example, anenergy set point change of more than 5%, a rep rate change of more than2%, or an interburst interval change of more than 10% would be requiredto trip the state transition. Also, the change would be required topersist for several bursts, to ensure that it was not just a one timechange, a calibration burst, or a wafer change.

[0139] State Common Code

[0140] All three states have in common the middle data processing loop,which is to compute burst average voltage and update the moving averageof burst average voltage. As a matter of implementation, computation ofthe burst average may be done by some other part of the LCP, sincerunning averages of various parameters are already maintained for otherreasons. In that case, the F₂ Inject algorithm would simple receive thenewest BAV value, update the moving average, and check to see if anyother action is necessary based on operating state.

[0141] Corner Cases

[0142] If customer were to change operating condition every time aninjection occurred, the voltage sensitivity factor would never getupdated

[0143] Multiple operating point changes in a row could be lost, sincethe New Op Point state does not check for op point changes. The correctbehavior might be to check for op point changes

Second Preferred F₂ Inject Algorithm

[0144] For Two Chamber

[0145] Applicants through their testing have determined that the firstpreferred algorithm described above performs very well for a singlechamber laser system. However, the algorithm assumes for a two-chamberMOPA system that the master oscillator can be slaved to the poweramplifier for purposes of F₂ injection. In practice Applicants havedetermined that this assumption is not a good one which means thatseparate control of the master oscillator F₂ injections should beprovided.

[0146] Applicants therefore have provided below a second preferredalgorithm specification for use in a two-chamber MOPA System. Thegeneral explanation entitled “Theory of Operations” preceding the firstpreferred algorithm specification is applicable to the description inthis second specification.

Second Preferred Algorithm Specification

[0147] Purpose

[0148] The purpose of this section is to provide a top-level state chartand detailed flow charts for each state to define all of the logic toimplement the algorithm outlined in the Theory of Operation section. Allequations and symbols are (hopefully) consistent between the twosections. Where appropriate, guides will be provided as to how thealgorithm should be implemented based on existing prototype code andsimulations.

[0149]FIG. 9A is a top-level state diagram, which provides a summaryview of the F₂ Inject algorithm. Each state will be discussedindividually in the following pages.

[0150] Common Code

[0151] As shown in FIG. 9B, this preferred, F₂ Inject algorithm relieson certain actions being performed between each burst. These actionsrelate to the bookkeeping functions that monitor Burst Average Voltageand Energy (both MO & PA), the current estimate of F₂ consumed duringthe cycle, whether the Operating Point has changed, and whether a ManualInjection has been requested. Regardless of what F₂ Injection mode(C152: Auto, ECI, Manual) is set, these tasks should be performed eachtime the F2 function is called. By pulling this common code out of thestate machine, it can be guaranteed that the laser control paddleprovides relevant F₂ information without requiring the injectionalgorithm to be running.

[0152] As mentioned previously, the natural granularity for thisalgorithm is at the burst level, since operating conditions are fixedwithin a burst. Between each burst, the burst average voltage must becomputed from the individual high voltage set points of all the pulsesin that burst. In addition, it is suggested that burst average energy becomputed for both the MO and PA chambers. This averaging provides somefiltering of the data and obviates the need to distinguish between bursttransient voltage, final voltage, or voltage target for a burst. Inprototype code, burst average voltage was computed as a one-lineaddition to the other pulse-to-pulse processing, and passed to a routinethat only executed between bursts. The burst average voltage is runthrough a moving average filter, which provides addition smoothing ofthe data. Voltage rise is computed at the end of each burst so that thevalue is available if an op point change or inject request is issued,either manually or automatically. Burst average energy and BAE movingaverage are computed in exactly the same way. These values will berequired in the near future to handle adjusting chamber F₂ injectionsindependently. Between each burst, a check is made to see if a ManualInjection has been issued externally, either from the laser controlpaddle or stepper/scanner interface. This may have been in response toan Injection Request from the algorithm, in which case it should betreated as an “OK” to perform an automatic inject.

[0153] The estimated amount of [F₂] consumed since the previousinjection is computed between each burst from a combination of shotcount (active consumption) and elapsed time (passive consumption). Thisvalue is available for each chamber on the paddle, and is used by F₂Inject state machine to determine if an automatic injection isnecessary.

[0154] The various operating states for the F₂ Inject algorithm arehandled by a simple state machine, which is called once per burst. Thereare only three F₂ operating states for the laser: Refill/Inject, FixedOp Point, and New Op Point. During normal operation, the laser willspend most of it's time in Fixed Op Point, running at some fixedoperating condition while the F₂ Inject algorithm simply monitors themoving average of burst average voltage. This state will be discussedfirst.

[0155] Fixed Op Point

[0156] Fixed Op Point portion of the algorithm (referring to “fixedoperation point”) as shown in FIG. 9C is a state that does very littleother than check for two conditions. First, if an Op Point change isdetected, a transition is made to “New Op Point.” That state handles theContinuity portion of voltage rise described in the Theory of Operation.Second, the most recent estimate [F₂] consumed is compared with theconsumption target for the current cycle. If more than the target amountof F₂ has been consumed, an injection is requested. Depending on thecustomer, the injection will begin either immediately (resulting in atransition to the Refill/Inject state), or the algorithm will wait untilthe request for injection has been accepted. In the later case, the F₂algorithm will continue to operate as normal in the “Fixed Op Point”state, monitoring voltage rise and [F₂] consumption. When the signal forInject is given by customer hardware, the transition to Refill/Injectwill occur. In this way, a correct accounting of F₂ consumed and voltagerise due to consumption will be maintained.

[0157] Refill/Inject

[0158] This Refill/Inject state of the algorithm as shown in FIGS. 9Dand 9E provides a starting point for each F₂ consumption cycle. On entryto this state, an [F₂] consumption target is set for the cycle, alongwith a value to be used as the active consumption rate. Both of thesevalues are fixed for the entire cycle.

[0159] The actual F₂ injection occurs while in the Refill/Inject state,as indicated by the small “Wait for F₂ Injection” block in the stateEntry code block. At this point, refer to the detailed diagram inanother document that describes how to actually perform an F₂ injectionfrom a nuts and bolts perspective—i.e., open Valve A, shoot gas in tube,close Valve A.

[0160] As can be seen from the state flow chart in FIGS. 9D and 9E,different actions are taken depending on whether the state was enteredfollowing a refill or an injection request. In the case of a refill, theconsumption target is equal to the (fixed) injection size and the ACR isread from persistent memory. If a consumption cycle has just completedand this state is entered for an injection, the consumption target iscomputed from a combination of the injection size and the estimated [F₂]remainder from the previous cycle. The voltage rise during the previouscycle is used to update the active consumption rate. This new ACR isthen used for the next cycle.

[0161] Note that two different methods are used to compute consumptionamount in this algorithm. During a consumption cycle, ACR times shotcount plus PCR times elapsed time is used to determine the amount of F₂consumed for the purpose of timing the next injection. In theRefill/Inject state, voltage rise times injection sensitivity is usedinstead. When the algorithm is converged, these values should all bemutually consistent. However, it is the job of the Inject state to findthe value of ACR that makes this true.

[0162] After the [F₂] target and ACR for the next cycle have beendetermined, the main purpose of the Refill/Inject state is to establishthe reference voltage against which voltage rise is measured. This isdone by monitoring the moving average of burst average voltage for somenumber of bursts (configurable, but probably O[50]). When the prescribednumber of bursts is exceeded, the reference voltage is set, and thestate exit code is run.

[0163] Following an inject, the exit code for this state updates theinternal estimate of F₂ consumption sensitivity, [F₂]_(□V). This valueis just the ratio of F₂ added to voltage drop. A simple low-pass filteris applied to the data to smooth out the measurement from one inject tothe next.

[0164] New Op Point

[0165] The New Op Point state part of the algorithm as shown in FIG. 9Fhandles changes in operating point—the combination of energy set point,rep rate, burst length, and interburst interval—which can throw off theconsumption rate estimator. Because a change in any one of theseparameters may result in a step change in burst average voltage, it isnecessary to correct for this effect in order to properly track voltagerise due to F₂ depletion. The basic mechanism is the same as that usedto determine the reference voltage in the Refill/Inject state. Movingaverage of burst average voltage is tracked for a prescribed number ofbursts. When this number is exceeded, the voltage change due to theoperating point change is computed, and this change is added to thereference voltage offset.

[0166] Referring back to the top-level state transition diagram, thereis a path from Refill/Inject to New Op Point. This path will be taken ifan operating point change occurs before a reference voltage has beenestablished immediately following an injection. In that case, thecorrect behavior is to not update the consumption sensitivityfactor—which would require a converged reference voltage—and to let NewOp Point handle establishing the reference voltage for the new cycle. Ineither case, New Op Point always transitions to Fixed Op Point uponsatisfying its exit condition.

[0167] There is also a valid path from New Op Point back to itself Inthe case of multiple back-to-back op point changes, the Bursts-In-Statecounter is reset, and a new attempt is made to compute the voltagechange. The old op point in this case will continue to the last validFixed Op Point. So long as the laser eventually lands in at a consistentnew operating point, only the last voltage change should matter.However, if this takes too much time, the underlying assumption that aninsignificant amount of F2 is consumed during the change would beviolated. A counter should be incremented to detect this condition. Ifthe counter exceeds some number in a row, voltage tracking should beabandoned for that inject cycle.

Implementation Notes

[0168] Convergence Exit Criterion

[0169] In the current algorithm specification, both the Refill/Injectand New Op Point states have an exit criterion that requires a fixedburst count to establish a new burst average voltage. A more refinedexit condition check would be to monitor the gradient of burst averagevoltage following either a Refill/Injection or op point change, anddeclare the appropriate reference voltage to be established when thegradient falls below a prescribed threshold. This would ensure that thealgorithm did not spend too long in either Refill/Inject or New OpPoint, and that the reference voltage used to determine voltage rise wasnot establish while voltage was still settling.

[0170] Op Point Change Detection

[0171] A simple function can be used to determine if any of the keyoperating point parameters have changed between bursts. In order toavoid spurious detections, prototype code set percentage changethresholds that would trigger New Op Point being called. For example, anenergy set point change of more than 5%, a rep rate change of more than2%, or an interburst interval change of more than 10% would be requiredto trip the state transition. Also, the change would be required topersist for several bursts, to ensure that it was not just a one timechange, a calibration burst, or a wafer change.

[0172] Confidence Estimate

[0173] When the laser is running at a fixed operating condition for along period of time, the voltage pattern typical of F2 depletionprovides a very consistent way to adaptively tune the active consumptionrate estimate. However, each time the laser changes operating conditionor sits idle, more error is introduced into the algorithm. One possibleway to account for this is through a simple confidence estimate. Foreach op point change, the confidence is reduced, say by 10%, from apossible starting confidence of 100%. When an injection is performed,the confidence is used to weight the newest ACR estimate. If confidenceis low—lots of op point changes, long off times, an inconsistent voltagepattern—the new estimate is given little weight when updating the systemACR. If confidence is high, the newest ACR is weighted more heavily.This feature will likely be a future revision to the algorithm.

[0174] Corner Cases

[0175] If customer were to change operating condition every time aninjection occurred, the voltage sensitivity factor would never getupdated.

Control of F₂ Concentration Based-On Timing

[0176] Applicants have discovered that the time difference between thezero Voltage crossing of the master oscillator Cp capacitor 82 as shownin FIG. 5A and MO light out is a strong function of only F₂concentration. Therefore this time difference can be used to monitor MOchamber F₂ concentration in the master oscillator and to control theconcentration at a desired level or within a desired range. FIG. 12Ashows at 100 a typical graph of the MO peaking capacitor voltage duringa discharge and a corresponding graph 102 of intensity of MO light out.Applicants measure the time difference between zero crossing 104 and thetime for crossing of a relative intensity (in this case 10% of intensitymaximum) as shown at 106. This delay time is labeled ΔT.

[0177]FIG. 12B shows the effect of changing the F₂ concentration of boththe PA and MO from 34 kPa to 40 kPa. FIG. 12C shows the effect ofchanging the MO F₂ concentration but keeping the PA F₂ concentrationconstant.

[0178] These data show that the delay data is a good indication of F₂concentration in the master oscillator and not a good indicator of F₂concentration in the PA. Applicants have shown through their experimentsthat charging voltage (and pulse energy), repetition rate and loadfactor have relatively very small effects on AT as compared to the MO F₂concentration. Therefore, a preferred method of controlling the F₂concentration in the master oscillator is to perform a series of teststo determine the F₂ concentration that produce the most desirable MOoutput properties and set the measured ΔT values for zero Cp to lightout a target concentration and then during operation control the F₂concentration within a desired range of that ΔT value. This can beprogrammed so that the control is fully automated using well-knowntechniques.

Atomic emission F₂ Monitor

[0179] Applicants have developed a spectroscopic method of measuring thefluorine concentration in ArF, KrF and F₂ lasers. This technique can beused to control the F₂ concentrations in the MO independent of the PA toassure that the MO chamber is operating with the most desirable F₂concentration.

[0180] A preferred embodiment is shown in FIG. 10A. In this casenormally wasted light reflected off the surface of the first beamexpansion prism 112A in the MO LNM as shown in FIG. 6 passes through asmall hole in the LNM casing and through a maximum reflection (at 248nm) mirror and is picked up by a filter optic pick-up and is transmittedto a small spectrometer such as a miniature filter optic spectrometeravailable from Ocean Optics, Inc. (The mirror designed to reflectspecific wavelength passes many other spectral bands). Thus, thespectrometer sees light from the entire discharge region. Preferably,the fluorine concentration is determined by the ration of intensity ofone or more atomic fluorine spectral lines as compared to one or moreatomic spectral line of a gas with a known concentration, preferably oneor more of the neon lines. FIGS. 10B and 10C shows some examples ofavailable fluorine and neon lines. In the FIGS. 10B and C charts the F₂concentration is 26.5 kPa and for the 10C chart the F₂ concentration is24 kPa. The 685.6 nm line was compared with the 692.9 nm Ne line. Asshown in FIG. 10D, the ratio of the ratios indicated that the fluorinein the FIG. 10B case was 1.113 greater than the fluorine in the FIG. 10Ccase. The actual ratio was 1.104. This suggests an error of aboutone-percent. The same set-up can be used to monitor F₂ concentration ina line narrowed ArF master oscillator chamber. A similar set-up can beused for line selected master oscillator in an F₂ MOPA system. Thistechnique can also be used in single chamber laser systems line narrowedand broad band.

Injection Based on Weak-Line, Strong-Line Ratio

[0181] As indicated above the basic laser system shown in FIG. 4 can beconfigured as an F₂ laser system operating with a 157.63 nm beam. Adetailed description of such a system is provided in U.S. patentapplication Ser. No. 10/243,101 which is incorporated by referenceherein. As described in that application, Applicants have discoveredthat the master oscillator of an F₂ MOPA system can be operated at F₂partial pressures low enough to virtually eliminate a bothersome weakfluorine spectral line at 157.52 nm so that a substantially pure outputbeam is produced at 157.63 nm, while at the same time producingsufficient output to adequately seed the power amplifier of the MOPAsystem. In the case of the F₂ laser systems; therefore, a preferredtechnique for controlling the MO F₂ partial pressure is to continuouslyor periodically monitor the two spectral lines and the output energy andto maintain partial pressure in the MO at levels such that the ratio ofthe strong line to the weak line is sufficiently large to meet bandwidth requirements and the MO pulse energy output is sufficiently largeto adequately seed the power amplifier.

Variable Bandwidth Control

[0182] As described above, this preferred embodiment of the presentinvention produces laser pulses much more narrow than prior art excimerlaser bandwidths. In some cases, the bandwidth is more narrow thandesired giving a focus with a very short depth of focus. In some cases,better lithography results are obtained with a larger bandwidth.Therefore, in some cases a technique for tailoring the bandwidth will bepreferred. Such a technique is described in detail in U.S. patentapplication Ser. Nos. 09/918,773 and 09/608,543, which are incorporatedherein by reference. This technique involves use of computer modeling todetermine a preferred bandwidth for a particular lithography results andthen to use the very fast wavelength control available with the PZTtuning mirror control shown in FIGS. 16B1 and 16B2 to quickly change thelaser wavelength during a burst of pulses to simulate a desired spectralshape. This technique is especially useful in producing relatively deepholes in integrated circuits.

Controlling Pulse Energy, Wavelength and Bandwidth

[0183] Prior art excimer lasers used for integrated circuit lithographyare subject to tight specifications on laser beam parameters. This hastypically required the measurement of pulse energy, bandwidth and centerwavelength for every pulse and feedback control of pulse energy andbandwidth. In prior art devices the feedback control of pulse energy hasbeen on a pulse-to-pulse basis, i.e., the pulse energy of each pulse ismeasured quickly enough so that the resulting data can be used in thecontrol algorithm to control the energy of the immediately followingpulse. For a 1,000 Hz system this means the measurement and the controlfor the next pulse must take less than 1/1000 second. For a 4000 Hzsystem speeds need to be four times as fast. A technique for controllingcenter wavelength and measuring wavelength and bandwidth is described inU.S. Pat. No. 5,025,455 and in U.S. Pat. No. 5,978,394. These patentsare incorporated herein by reference. Additional wavemeter details aredescribed in co-owned patent application Ser. No. 10/173,190 which isalso incorporated by reference herein.

[0184] Control of beam parameters for this preferred embodiment is alsodifferent from prior art excimer light source designs in that thewavelength and bandwidth of the output beam is set by conditions in themaster oscillator 10 whereas the pulse energy is mostly determined byconditions in the power amplifier 12. In a preferred embodiment,wavelength bandwidths is measured in the SAM 9. This equipment in theSAM for measuring bandwidth utilizes an etalon and a linear diode arrayas explained in the above-referenced patents and patent applications.However an etalon with a much smaller free spectral range is utilized inorder to provide much better image resolution. Pulse energy is monitoredin both the LAM and the SAM and may also be monitored at the scanner. Ineach case using pulse energy monitors as described in the above patentsand patent applications. These beam parameters can also be measured atother locations such as the output of the power amplifier, the output ofthe master oscillator and the input to the stepper/scanner.

Pulse Stretcher

[0185] Integrated circuit scanner machines comprise large lenses whichare difficult to fabricate and costs millions of dollars. These veryexpensive optical components are subject to degradation resulting frombillions of high intensity and ultraviolet pulses. Optical damage isknown to increase with increasing intensity (i.e., light power(energy/time) per cm² or mJ/ns/cm²) of the laser pulses. The typicalpulse length of the laser beam from these lasers is about 20 ns so a 5mJ beam would have a pulse power intensity of about 0.25 mJ/ns.Increasing the pulse energy to 10 mJ without changing the pulse durationwould result a doubling of the power of the pulses to about 0.5 mJ/nswhich could significantly shorten the usable lifetime of these expensiveoptical components. The Applicants have avoided this problem byincreasing substantially the pulse length from about 20 ns to more than50 ns providing a reduction in the rate of scanner optics degradation.This pulse stretching is achieved with pulse stretcher unit 12 as shownin FIG. 4. A beam splitter 16 reflects about 60 percent of the poweramplifier output beam 14B into a delay path created by four focusingmirrors 20A, 20B, 20C and 20D. The 40 percent transmitted portion ofeach pulse of beam 14B becomes a first hump of a corresponding stretchedpulse in of beam 14C. The stretched beam 14C is directed by beamsplitter 16 to mirror 20A which focuses the reflected portion to point22. The beam then expands and is reflected from mirror 20B whichconverts the expanding beam into a parallel beam and directs it tomirror 20C which again focuses the beam again at point 22. This beam isthen reflected by mirror 20D which like the 20B mirror changes theexpanding beam to a light parallel beam and directs it back to beamsplitter 16 where 60 percent of the first reflected light is reflectedperfectly in line with the first transmitted portion of this pulse inoutput beam 14C to become most of a second hump in the laser pulse. The40 percent of the reflected beam transmits beam splitter 14 and followsexactly the path of the first reflected beam producing additionalsmaller humps in the stretched pulse. The result is stretched pulse 14Cwhich is stretched in pulse length from about 20 ns to about 50 ns.

[0186] The stretched pulse shape with this embodiment has two largeapproximately equal peaks 13A and 13B with smaller diminishing peaksfollowing in time the first two peaks. The shape of the stretched pulsecan be modified by using a different beam splitter. Applicants' havedetermined that a beam splitter reflecting about 60 percent produces themaximum stretching of the pulse as measured by a parameter known as the“time integrated square” pulse length or “TIS”. Use of this parameter isa technique for determining the effective pulse duration of pulseshaving oddly shaped power vs. time curves. The TIS defined as:$t_{IS} = \frac{\left( {\int{{I(t)}{t}}} \right)^{2}}{\int{{I^{2}(t)}{t}}}$

[0187] Where I(t) is the intensity as a function of time.

[0188] In order to maintain the beam profile and divergence properties,the mirrors that direct the beam through the delay propagation path mustcreate an imaging relay system that also should act as a unity,magnification, focal telescope. The reason for this is because of theintrinsic divergence of the excimer laser beam. If the beam weredirected through a delay path without being imaged, the beam would be adifferent size than the original beam when it is recombined at the beamsplitter. To create the imaging relay and a focal telescope functions ofthe pulse stretcher the mirrors are designed with a specific radius ofcurvature which is determined by the length of the delay path. Theseparation between mirrors 20A and 20D is equal to the radius ofcurvature of the concave surfaces of the mirrors and is equal to{fraction (1/4)} the total delay path.

[0189] The relative intensities of the first two peaks in the stretchedpulse can be modified with the design of the reflectivity of the beamsplitter. Also, the design of the beam splitter and therefore the outputTIS of the pulse stretcher are dependent upon the efficiency of the beamrelay system and therefore the output TIS is also subject to the amountof reflectivity of the imaging relay mirrors and the amount of loss atthe beam splitter. For an imaging relay mirror reflectivity of 97% and aloss of 2% at the beam splitter, the maximum TIS magnification occurswhen the reflectivity of the beam splitter is 63%.

[0190] The alignment of the pulse stretcher requires that two of thefour imaging relay mirrors be adjustable. Each of the two adjustablemirrors would have tip/tilt adjustment creating a total of four degreesof freedom. It is necessary that the two adjustable mirrors be locatedat opposite ends of the system because of the confocal design of thesystem. To create a self-aligning pulse stretcher would requireautomated adjustment of the necessary four degrees of freedom and adiagnostic system which could provide feedback information tocharacterize the alignment. The design of such a diagnostic system,which could qualify the alignment performance, would require an imagingsystem capable of imaging both the near field and far field output ofthe pulse stretcher. By examining the overlay of the sub-pulses with theoriginal pulse at two planes (near field and far field) one would havethe necessary information to automatically adjust the mirrors to producean output where each of the sub-pulses propagate in a co-linear mannerwith the original pulse.

Relay Optics

[0191] In this preferred embodiment the output beam 14A of the masteroscillator 8 is amplified by two passes through power amplifier 10 toproduce output beam 14B. The optical components to accomplish this arecontained in three modules which Applicants have named: masteroscillator wave front engineering box, MO WEB, 24, power amplifierwavefront engineering box, PA WEB, 26 and beam reverser, BR, 28. Thesethree modules along with line narrowing module 8B and output coupler 8Aare all mounted on a single vertical optical table independent ofdischarge chamber 8C and the discharge chamber of power amplifier 10.Chamber vibrations caused by acoustic shock and fan rotation must beisolated from the optical components.

[0192] The optical components in the master oscillator line narrowingmodule and output coupler are in this embodiment substantially the sameas those of prior art lithography laser light sources referred to in thebackground section. The line narrowing module includes a three or fourprism beam expander, a very fast response tuning mirror and a gratingdisposed in Litrow configuration. The output coupler is a partiallyreflecting mirror reflecting 20 percent of the output beam for KrFsystems and about 30 percent for ArF and passing the remainder. Theoutput of master oscillator 8 is monitored in line center analysismodule, LAM, 7 and passes into the MO WEB 24. The MO WEB contains atotal internal reflection (TIR) prism and alignment components forprecisely directing the output beam 14A into the PA WEB. TIR prisms suchas the one shown in FIG. 3A can turn a laser beam 90 degrees with morethan 90 percent efficiency without need for reflective coatings whichtypically degrade under high intensity ultraviolet radiation.Alternatively, a first surface mirror with a durable high reflectioncoating could be used in place of the TIR prism.

[0193] The PA WEB 26 contains a TIR prism and alignment components (notshown) for directing laser beam 14A into a first pass through poweramplifier gain medium. Alternatively, as above a first surface mirrorwith a high reflection coating could be substituted for the TIR prism.The beam reverser module 28 contains a two-reflection beam reversingprism relies on total internal reflection and therefore requires nooptical coatings. The face where the P-polarized beam enters and exitsthe prism is oriented at Brewster's angle to minimize reflection lasers,making the prism almost 100% efficient.

[0194] After reversal in the beam reversing module 28, partiallyamplified beam 14A makes another pass through the gain medium in poweramplifier 10 and exits through spectral analysis module 9 and PA WEB 26as power amplifier output beam 14B. In this embodiment the second passof beam 14A through power amplifier 10 is precisely in line with theelongated electrodes within the power amplifier discharge chamber. Thefirst pass follows a path at an angle of about 6 milliradians relativeto the path of the second pass and the first path of the first passcrosses the center line of the gain medium at a point half way betweenthe two ends of the gain medium.

Beam Expansion Prisms

[0195] Coming out of the PA, the fluence of the beam is higher thananywhere else in the system (due to small beam size and high pulseenergy). To avoid having such high fluence incident on the opticalcoatings in the OPuS module, where coating damage could result, beamexpansion prisms were designed into the PA WEB. By expanding thehorizontal beam width by a factor of 4, the fluence is reduced to{fraction (1/4)} its previous level.

[0196] The beam expansion is accomplished using a pair of identicalprisms with 20° apex angle.

[0197] The prisms are made of ArF-grade calcium fluoride and areuncoated. By utilizing an incidence angle of 68.6° on each prism,anamorphic magnification of 4.0 is achieved, and the nominal deviationangle of the pair is zero. The total Fresnel reflection loss from thefour surfaces is about 12%.

Beam Delivery Unit

[0198] In this preferred embodiment a pulsed laser beam meetingrequirements specified for the scanner machine 2 is furnished at thelight input port of the scanner. A beam analysis module as shown at 38in FIG. 4 called a BAM is provided at the input port of the scanner tomonitor the incoming beam and providing feedback signals to the lasercontrol system to assure that the light provided to the scanner is atthe desired intensity, wavelength, bandwidth, and complies with allquality requirements such as dose and wavelength stability. Wavelength,bandwidth and pulse energy are monitored by meteorology equipment in thebeam analysis module on a pulse to pulse basis at pulse rates up to4,000 Hz using techniques described in U.S. patent application Ser. No.10/012,002 which has been incorporated herein by reference.

[0199] Other beam parameters may also be monitored at any desiredfrequency since these other parameters such as polarization, profile,beam size and beam pointing are relatively stable, may be normallymonitored much less frequently than the wavelength, bandwidth and pulseenergy parameters.

[0200] This particular BDU comprises two beam-pointing mirrors 40A and40B one or both of which may be controlled to provide tip and tiltcorrection for variations beam pointing. Beam pointing may be monitoredin the BAM providing feedback control of the pointing of one or both ofthe pointing mirrors. In a preferred embodiment piezoelectric driversare provided to provide pointing response of less than 7 milliseconds.

Special F₂ Laser Features

[0201] Most of the above descriptions generally apply directly to an ArFlaser system but almost all of the features are equally applicable toKrF lasers with minor modifications which are well known in theindustry. Some significant modifications are required, however, for theF₂ version of this invention. These changes could include a lineselector in the place of the LNP and/or a line selector between the twochambers or even downstream of the power amplifier. Line selectorspreferably are a family of prisms. Transparent plates properly orientedwith respect to the beam could be used between the chambers to improvethe polarization of the output beam. A diffuser could be added betweenthe chambers to reduce the coherence of the output beam.

[0202] Various modifications may be made to the present inventionwithout altering its scope. Those skilled in the art will recognize manyother possible variations.

[0203] For lithography either ArF, KrF or F₂ systems could be utilized.This invention may be utilized for F2 injection in single chamber lasersystems such as the laser system described in U.S. Pat. No. 6,151,349.The algorithm described in FIGS. 8A-8F are preferred for a singlechamber system. This invention may also be applied to uses other thanlithography in which other ultraviolet wavelength may be needed. Animportant improvement here is the addition of equipment to a lasersystem to deliver an ultraviolet laser beam having desire beam qualitiesto an input port of a equipment needing an ultraviolet laser lightsource. Various feedback control arrangements other than those referredto herein could be used.

[0204] The reader should understand that at extremely high pulse ratesthe feedback control on pulse energy does not necessarily have to befast enough to control the pulse energy of a particular pulse using theimmediately preceding pulse. For example, control techniques could beprovided where measured pulse energy for a particular pulse is used inthe control of the second or third following pulse. Many other laserlayout configurations other than the one shown in FIG. 1 could be used.For example, the chambers could be mounted side-by-side or with the PAon the bottom. Also, the second laser unit could be configured as aslave oscillator by including an output coupler such as a partiallyreflecting mirror. Other variations are possible. Fans other than thetangential fans could be used. This may be required at repetition ratesmuch greater than 4 kHz. The fans and the heat exchanger could belocated outside the discharge chambers.

[0205] Accordingly, the above disclosure is not intended to be limitingand the scope of the invention should be determined by the appendedclaims and their legal equivalents.

We claim:
 1. A very narrow band high repetition rate gas discharge lasersystem comprising: A) a discharge chamber containing; a) a first lasergas and b) a first pair of elongated spaced apart electrodes defining afirst discharge region, c) a first fan for producing sufficient gasvelocities of said first laser gas in said first discharge region toclear from said first discharge region, following each pulse,substantially all discharge produced ions prior to a next pulse whenoperating at a repetition rate in the range of 4,000 pulses per secondor greater, d) a first heat exchanger system capable of removing atleast 16 kw of heat energy from said first laser gas, B) a linenarrowing unit for narrowing spectral bandwidths of light pulsesproduced in said first discharge chamber; C) a pulse power systemconfigured to provide electrical pulses to said first pair of electrodesand to said second pair of electrodes sufficient to produce laser pulsesat rates of about 4,000 pulses per second or greater with preciselycontrolled pulse energies in excess of about 5 mJ; D) a fluorineinjection system for maintaining, within desired ranges, fluorine gasconcentrations contained in laser chamber; said injection systemcomprising one or more processors programmed with one or more algorithmsfor monitoring ΔE/ΔV decrease during operation of the laser system andmonitoring ΔE/ΔV increase as a result of F₂ injections of knownquantities of F₂; E) a laser beam measurement and control system formeasuring pulse energy, wavelength and bandwidth of laser output pulsesproduced by said two chamber laser system and controlling said laseroutput pulses in a feedback control arrangement.
 2. The laser system asin claim 1 wherein said one or more algorithms comprise means formonitoring an active F₂ consumption rate and a means for monitoring apassive consumption.
 3. The system as in claim 1 wherein said ΔE/ΔVincrease is determined from a change in discharge voltage correspondingto a fixed energy output, just prior to and just after an F₂ injection.4. The system as in claim 1 wherein said ΔE/ΔV increase is determinedfrom a change in ΔE/ΔV based on values of ΔE/ΔV measured just prior toand just after an F₂ injection.
 5. The system as in claim 1 wherein saidvalues of voltage used to determine ΔV are represented by averageaverage voltage values in a plurality of pulses comprising one or moreburst of pulses said average voltage values defining a BAV.
 6. Thesystem as in claim 1 wherein said algorithms comprises a means to filtersaid BAV through an N-burst moving window.
 7. The system as in claim 5wherein said values of: ΔV _(inject) =BAV _(pre-inject) −BAV_(post-inject)
 8. The system as in claim 1 wherein said algorithmcomprises a means for utilizing a consumption sensitivity factor toprovide a low pass filter in calculating estimates of F₂ consumption. 9.The system as in claim 8 wherein said low pass filter is in the form of:[F₂]_(Δ  V) = [F₂]_(Δ  V) + k × ([F₂]_(Δ  V)^(last) − [F₂]_(Δ  V))


10. The system as in claim 1 wherein said algorithm comprises a meansfor utilizing a low pass filter to update values of active F₂consumption rates.
 11. The system as in claim 10 wherein said low passfilter is in the form of: ACR=ACR+k×(ACR ^(last) −ACR)
 12. The system asin claim 1 wherein said the voltage increase during operation isdetermined by calculating the difference between a value representingburst average voltage and a reference voltage wherein the referencevoltage is determined at the start of consumption cycles.
 13. The systemas in claim 2 wherein said algorithm comprises provisions for countingdischarges between fluorine injections and for monitoring time durationsbetween injections.
 14. A system as in claim 3 wherein said firstcontrol algorithm also comprises provisions for computing consumptionsensitivity factors which are a function of fluorine inject sizes andvoltage changes resulting from a specific fluorine injections.
 15. Asystem as in claim 3 wherein said algorithm comprises provisions forcomputing consumption sensitivity factors which are a function offluorine inject sizes and square roots of a voltage changes.
 16. A verynarrow band two chamber high repetition rate gas discharge laser systemcomprising: A) a first laser unit comprising: 1) a first dischargechamber containing; e) a first fluorine containing laser gas and f) afirst pair of elongated spaced apart electrodes defining a firstdischarge region, g) a first fan for producing sufficient gas velocitiesof said first laser gas in said first discharge region to clear fromsaid first discharge region, following each pulse, substantially alldischarge produced ions prior to a next pulse when operating at arepetition rate in the range of 4,000 pulses per second or greater, h) afirst heat exchanger system capable of removing at least 16 kw of heatenergy from said first laser gas, B) a line narrowing unit for narrowingspectral bandwidths of light pulses produced in said first dischargechamber; C) a second discharge chamber comprising: 1) a second fluorinecontaining laser gas, 2) a second pair of elongated spaced apartelectrodes defining a second discharge region 3) a second fan forproducing sufficient gas velocities of said second laser gas in saidsecond discharge region to clear from said second discharge region,following each pulse, substantially all discharge produced ions prior toa next pulse when operating at a repetition rate in the range of 4,000pulses per second or greater, 4) a second heat exchanger system capableof removing at least 16 kw of heat energy from said second laser gas; D)a pulse power system configured to provide electrical pulses to saidfirst pair of electrodes and to said second pair of electrodessufficient to produce laser pulses at rates of about 4,000 pulses persecond with precisely controlled pulse energies in excess of about 5 mJ;and E) a fluorine injection system for maintaining, within desiredranges, fluorine gas concentrations contained in laser chamber; saidinjection system comprising one or more processors programmed with analgorithm for monitoring ΔE/ΔV decrease during operation of the lasersystem and monitoring ΔE/ΔV increase as a result of F₂ injections ofknown quantities of F₂; F) relay optics for directing laser beamsproduced in said first laser unit through said second discharge chamberto produce an amplified output beam; G) a laser beam measurement andcontrol system for measuring pulse energy, wavelength and bandwidth oflaser output pulses produced by said two chamber laser system andcontrolling said laser output pulses in a feedback control arrangement.17. The system as in claim 16 wherein said one or more controlalgorithms comprise provisions for counting discharges between fluorineinjections and for monitoring time durations between injections.
 18. Thesystem as in claim 17 wherein said one or more control algorithms alsocomprise provisions for computing consumption sensitivity factors whichare a function of fluorine inject sizes and voltage changes resultingfrom a specific fluorine injections.
 19. The system as in claim 17wherein said one or more control algorithms comprise provisions forcomputing consumption sensitivity factors which are a function offluorine inject sizes and square roots of a voltage changes.
 20. Thesystem as in claim 18 wherein said one or more control algorithms areused to control F₂ concentrations in said first and second dischargechamber.
 21. The system as in claim 16 wherein said one or more controlalgorithms control fluorine concentration in said first dischargechamber separate from said second discharge chamber.
 22. The system asin claim 21 and further comprising a fluorine monitoring means formonitoring fluorine concentration in said first discharge chamber. 23.The system as in claim 22 wherein said fluorine monitoring meanscomprises a spectrometer.
 24. The system as in claim 23 wherein saidfirst discharge chamber is a master oscillator comprising a linenarrowing module and further comprising a light gathering means formonitoring waste light from said first discharge region and reflectedfrom an optical component in the line narrowing module.
 25. The systemas in claim 22 wherein said monitoring means comprises a timing meansfor monitoring a time differential representing a time differencebetween a high voltage pulse on a high voltage capacitor in said pulsepower system and a light pulse produced in said first discharge chamber.26. The system as in claim 25 wherein said time difference is based on azero voltage crossing in said high voltage capacitor and a lightintensity level crossing in said light pulse.
 27. A system as in claim16 and further comprising a beam delivery unit for delivering laserbeams to a lithography device.
 28. A system as in claim 16 and furthercomprising a pulse stretcher unit.